The invention pertains to a method and a device for tempering preforms in a blow-molding machine with a heating channel comprising reflective walls, the channel being formed by a heating box. The preforms are conducted through this heating channel so that they can be tempered to the blow-molding temperature. The heating box comprises heating means for heating the preforms. The heating box also comprises a cooling device comprising at least one blower. The cooling device serves to cool the reflective walls of the heating channel, in particular by means of a quantity of air flowing along the walls only outside the heating channel, wherein the heating output of the heating means is adjustable.
Such methods and devices are known from the prior art and are used, for example, in blow-molding machines sold by the applicant for the thermal conditioning of preforms prior to the blow-molding process.
The goal of such methods and devices is to bring the preforms, usually produced by injection molding, to the proper temperature for the following blow-molding step. In the case of PET, for example, this temperature is approximately 120° C. and must be maintained within a narrow range to avoid changes to the material resulting from overstretching or phase transitions of the material. The preforms are also usually given a certain temperature profile.
During the operation of such methods and devices, the preforms are heated by, for example, the radiant heat produced by radiant heaters. IR heaters and/or NIR heaters are preferably used. Radiation passing between the preforms or radiation not absorbed on passage through the preform is thrown back by the reflective walls of the heating channels to increase the efficiency of the heating boxes and to improve the depthwise heating of the preform material. As an unavoidable result, the reflective walls of the heating channels also heat up.
The walls of the heating channels usually consist of aluminum materials, which are good reflectors of radiation in the IR and/or NIR range. At temperatures above 180° C., however, such materials tend to corrode more readily and must therefore be cooled.
Methods and devices are known in which the preforms, as they are transported through the heating section formed, for example, by several heating channels arranged in succession in the transport direction, are subjected to cooling air, which is supplied through openings in the walls. This type of heating device of a blow-molding machine is described in, for example, US 2011/0300497 A1. In this blow-molding machine, the cooling air discharged toward the preforms has the primary effect of cooling the preforms, which is, after all, the purpose of the cooling. Air flowing past the preforms also acts on the walls of the radiant heater and thus also cools them. The heating output of the radiant heater and the cooling air output are specified permanently in an advance to achieve the desired temperature profile of the preforms.
In the case of the method and devices of the class in question, the desired temperature profile of the preforms is achieved by the appropriate adjustment of the radiant heaters. Active cooling of the area of the preforms to be tempered is not usually necessary, which means that there is no need to supply cooling air into the heating channel. It is known from the prior art, however, that the neck area of the preform can be actively cooled.
In the case of methods and devices of the class in question, the reflective walls are usually cooled by passing cooling air along them. To avoid an overheating of the reflector walls reliably, the cooling is adjusted in such a way that the critical temperature is not reached at maximum heating output of the radiant heaters. It is known that the cooling output can be manually adjusted when necessary.
Because different preforms with different temperature behavior are processed in blow-molding machines and because they are processed at different speeds, the heating output of the heating means of the heating channel must be adjustable. In the prior art, such heating is subjected to open-loop or closed-loop control as a function of various parameters. During operation of the blow-molding machine, all of the heating means are never set at their maximum output. Because the previously known cooling devices, however, are configured and operated in such a way that overheating of the reflective walls is prevented at all times, i.e., even in the case of maximum heating output, the cooling output is overdimensioned with respect to the heating output actually supplied during operation of the machine.
Heat is obviously withdrawn from the heating channel by the cooling device, as a result of which the efficiency of the heating device is decreased, wherein the relative heat losses becomes especially large precisely at reduced heating output levels.